The present invention is directed to a method for preparing colloidal dispersions of precious metal nanoparticles and to a method for isolating such precious metal nanoparticles from these dispersions. The methods disclosed herein describe the preparation of highly concentrated dispersions of nano-size precious metal particles by reducing the corresponding ions in aqueous alkaline solutions with polysaccharides and the subsequent isolation of such particles.
Colloidal dispersions of precious metal nanoparticles (as well as the nanoparticles isolated from such dispersions) are gaining importance in electronic applications; for example for the generation of conductive lines and patterns. They further find use in catalyst applications, for example for the preparation of core/shell type catalyst materials. Furthermore, they can be used in medical and therapeutical as well as in decorative applications.
The term “nanoparticle” as used in the context of this invention refers to particles with a medium particle size in the range of <200 nm (<0.2 micron) as determined by conventional electron microscopy methods (SEM/TEM).
In recent years, fine metallic particles, particularly nanoparticles of definite shape and size have received considerable interest, and attention because of their fascinating properties and potential applications, e.g. in semiconductors, consumer products, opto-electronics, electronics, catalysis, transportation, energy, medical sciences and biotechnology. The intrinsic properties of fine metallic particles are mainly determined by their size, shape, composition, crystallinity and structure.
A number of techniques have been proposed for the preparation of fine precious metal particles, including alcohol reduction, the polyol process, sonochemical methods, decomposition of organometallic precursors, vaporisation-condensation methods and electrolysis of bulk metals. Generally, precious metal particles are prepared in a reduction process employing reducing agents such as organic acids, alcohols, polyols, aldehydes, sugars etc. (ref to D. V. Goia, E. Matijevic, New. J. Chem. 1998, pages 1203-1215). In this reduction process, a suitable precious metal compound is reduced in an acidic or alkaline environment to the metal with oxidation state zero (0). The chemical reducing agents commonly used are toxic and/or carcinogenic compounds (e.g. hydrazine, sodium borohydride, formaldehyde) and cause safety and health problems in volume production.
In the well known polyol process, silver nanoparticles are prepared by the reduction of silver nitrate with ethylene glycol at about 160° C. The ethylene glycol serves as reductant and solvent. Typically, stabilizing/dispersing agents such as polyvinylpyrolidone (PVP) are employed (ref to Y. Sun and Y. Xia, Science, Vol. 298, 2176-2179 (2002)). The drawbacks with this process are the high energy consumption, the use of expensive organic glycol solvent and the recycling of waste solvent after use.
US 2002/0034675 is directed to precious metal nanoparticles, which are embedded in an aqueous solution of a temporary stabilizer. The nanoparticles are manufactured by reduction of chloride-free precursor compounds in water in the presence of a polysaccharide functioning as a stabilizer. Reducing agents such as hydrogen, hydrazine or ethanol are applied.
EP 796 147 B1 discloses surfactant-stabilized colloids of mono- and bimetallic particles of the groups VIII and IB of the Periodic System of the Elements (PSE) having particle sizes in the range of 1 to 10 nm. They are prepared by a reduction process in the presence of strongly hydrophilic surfactants. Chemical reducing agents such as hydrides, hydrogen or alkali formiates are applied.
WO 2007/112926 teaches a process for manufacture of silver-based particles via an intermediate silver (+1)-oxide species. Due to the presence of an organic dispersing agent, the silver (+1)-oxide species is thermally instable and decomposes to metallic silver upon heating to temperatures in the range of 45 to 90° C. This process is a two-step process and therefore time-consuming and costly.
US 2006/0090598 A1 discloses an aqueous-based method for producing ultra-fine silver powders by reducing a silver-ammonia complex with glucose and arabic gum. A similar route for manufacturing of highly dispersed silver nanoparticles by reducing the silver ammonia complex [Ag(NH3)2]+ with glucose in the presence of a stabilizing agent was reported recently; ref to D. Andreescu, C. Eastman. K. Balantrapu and D. Goia, J. Mater. Res., Vol. 22, No. 9, 2488-2495 (2007). This process, employing a reducing agent as well as a stabilizing agent, yields silver particles with an average particle size of about 30-120 nm.
U.S. Pat. No. 5,248,772 describes the formation of colloidal metal dispersions using aminodextrans as reductants and dispersing agents. Colloidal metal particles, preferably gold and silver particles, having a crosslinked aminodextran coating with pendent amine groups attached thereto are generated. Such coated particles can be used as markers in immunological and biological assays and as therapeutic agents. The process described in the patent is suitable for highly diluted precious metal salt solutions (typical concentrations in the range of 0.2 to 0.84 mMol metal/1). The precious metal salt solutions (HAuCl4 and AgNO3) are used as received; thus the aminodextrane is applied in acidic environment and no adjustment of the pH is conducted. Due to the low precious metal concentrations employed, the process suffers from a low yield.
As a result, the presently known processes for preparation of precious metal nanoparticle dispersions and nanoparticles therefrom are not satisfactory in terms of cost, yield, process simplicity, environmental safety, and energy consumption.
It was therefore an objective of the present invention to provide improved methods for the manufacture of precious metal nanoparticle dispersions and for the isolation of the precious metal nanoparticles therefrom. The methods should offer high yields and should be versatile, simple, straight-forward, environmentally friendly, cost-efficient and energy-saving. This objective can be met by the methods of the present invention.